No, that's indeed not an explanation as that wire is permanently tied to ground in my manual car. Same goes for the airconditioning inputs as my car doesn't have one.

At speed, I seem to remember that - while decelerating - only the speed and clutch had an effect on how the IAC was controlled.
The pinout diagram mentions the information of a closed TPS is either used to start a special idling program or is relevant during fuel shut off, so at standstill/low speed or during engine braking.

I had already inspected the insides of the ECU and EZK. On the ECU I cleaned the board around the opamp as it had some white residue, presumably flux residue. The rest of the board looked absolutely immaculate. I was impressed by the solder quality (and I was in that industry for nearly 19 years, so I recognize a good solder joint when I see one). Despite all this, I resoldered every through hole component just to exclude the possibility of a microscopically cracked joint.
The only part I replaced was the main 470uF/40V electrolytic as its ESR measured a bit high (to be expected at its age).

I don't think my car should have the ECU of the B230F, but it does. Now I have one of a B200F coming my way, and I wonder if this has any other effect than hopefully curing the idling speed issue. To be continued.